Hitherto, a fuser includes a heat roller and a belt arranged to be wound around a part of the heat roller. The heat roller is also called a fixing roller. Plural rollers are arranged inside the belt. Although the belt is wound around the heat roller by the rollers arranged inside the belt, when a firm sheet such as a thick paper is fed, there is case where a space is generated between the heat roller and the belt due to the firmness of the sheet, and a floating phenomenon of the belt occurs. When the belt floats, adhesion between the heat roller and the sheet can not be maintained, and there is a problem that defective fixing occurs.